The present invention relates generally to the field of disc drive devices and more particularly, but without limitation, to improving operational performance of a disc drive by detecting defects in servo burst patterns used to control the position of a disc drive head.
Modern hard disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant, high speed. Data are stored on the discs in a plurality of concentric circular tracks by an array of transducers (xe2x80x9cheadsxe2x80x9d) mounted to a radial actuator for movement of the heads relative to the discs.
A voice coil motor (VCM) is used to position the heads with respect to the disc surfaces. The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from an actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
The VCM includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When current is applied to the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move relative to the permanent magnets, causing the actuator body to pivot about the pivot shaft and move the heads across the disc surfaces.
Head positional control is typically achieved with a closed loop servo circuit such as disclosed in U.S. Pat. No. 5,262,907 issued to Duffy et al., assigned to the assignee of the present invention. Such a servo circuit utilizes servo data written to the discs during the disc drive manufacturing process to detect and control the position of the heads through the generation of a position error signal (PES) which is indicative of the position of the head with respect to a selected track. The PES is presented as a position dependent signal having a magnitude indicative of the relative distance between the head and the center of a track and a polarity indicative of the direction of the head with respect to the track center. Thus, it is common for the PES to have normalized values corresponding to a range of, for example xe2x88x921.0 to +1.0, as the head is swept across a selected track and to have a value corresponding to a value of 0 when the head is positioned over the center of the track. As will be recognized, modern servo circuits typically generate the PES as a sequence of digital samples which generally correspond to the above analog range.
The PES is generated by the servo circuit by comparing the relative signal strengths of burst signals obtained from precisely located magnetized servo burst patterns in the servo data on the disc surface. The burst patterns are generally arranged in an offset pattern so that, through sampling and algebraic combination of the burst signals, the relative position of the head to a particular track center can be determined and controlled.
More particularly, digital representations of the analog burst signals are provided to a servo microprocessor (such as a digital signal processor), which obtains a digital representation of the value of the PES from a selected combination of samples from the analog burst signals. The microprocessor then compares the value of the PES to a desired value indicative of the desired position of the head to the selected track and issues a digital correction signal to a coil driver, which in turn provides an analog current to the actuator coil to adjust the position of the actuator.
It will be recognized that accurate control of the position of the heads is of paramount importance in the reliable reading and writing of data to the discs. The servo circuit generally attempts to maintain the head over the center of the selected track so as to minimize the potential for overwriting data on adjacent tracks or having the magnetization of adjacent tracks interfere with the reading of the data stored on the selected track. Thus, it is common during read and write operations to compare the absolute value of each PES sample to a predetermined safe-threshold value in order to assure the head is correctly positioned relative to the track. Should the value of a particular PES sample exceed the threshold, the read or write operation is temporarily suspended until the PES is brought back down to a safe value. These thresholds are referred to as xe2x80x9cread-faultxe2x80x9d and xe2x80x9cwrite faultxe2x80x9d thresholds and will generally range from about 10% to 20% of the width of the track (as measured from the center of the track).
A selected PES sample may have a value that exceeds the safe-threshold value during a read or write operation for a variety of reasons. One such reason is that the head is actually positioned off track center a distance sufficient to exceed the threshold value; particularly, it will be recognized that mechanical shocks supplied to the disc drive during operation can result in movement of the head away from the center of the selected track (sometimes referred to as an off-track condition). As a result, it is desirable to suspend the read or write operation until such off-track condition can be corrected.
Another reason that a selected PES sample may have a value that exceeds the safe-threshold value is the existence of a localized defect in the servo data associated with the PES sample; in such a case, the head may be correctly located with respect to the track, but the reported PES sample erroneously indicates otherwise. Such a defect in the servo data can occur as a result of a localized anomaly in the media on the surface of a disc, so that the media does not possess the necessary magnetic properties to allow the servo data to be properly written at this location. Undetected errors can also occur during the servo track writing process during manufacture of the disc drive, so that incorrect servo data are provided to the disc at a particular location.
Heads that utilize magneto-resistive (MR) technology can also be affected by a phenomenon known as a thermal asperity, which can induce significant distortion in a readback signal. An MR head utilizes a read element that undergoes change in electrical resistance in the presence of a magnetic field of selected orientation. By passing a bias current through an MR read element, the selective magnetization of the disc can be detected in relation to changes in voltage across the element.
However, very small defects on the surface of the recording discs can be large enough to physically contact the MR read element of the heads as the discs rotate under the heads. Such contact, while of very short time duration, results in frictional heating of the MR read element and the change of temperature brought about by the contact also produces a change in resistance in the MR element, distorting the readback signal. A similar effect can occur when the head contacts particulate contamination on the disc surface.
Small xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d in the disc surfaces can also induce thermal asperity events even without physical contact between the MR element and the disc surface. Because the bias current applied to the MR element results in heating of the MR element, a thermal equilibrium is established in which the generated heat in the MR element is constantly dissipated from the MR element through other elements of the head assembly and, to a lesser extent, across the air bearing supporting the slider to the disc itself. Thus, disc surface variations that change the spacing between the MR element and the disc can induce attendant changes in the heat dissipation characteristics of the head, resulting in distortion in the readback signal obtained from the head.
Regardless of the source of the defect in the servo data, such a defect is typically manifested as a one sample error in the sequence of PES samples. The erroneous PES sample does not provide a true indication of head position relative to the center of the selected track, and further, if the erroneous PES sample is interpreted by the servo circuit as an impulse function, an unwanted oscillatory response will be induced into the system.
Because of the problems associated with defects in the servo data, it is desirable to provide a servo circuit which can detect the presence of a defective servo burst pattern and take the necessary steps to reduce the effects of such upon the operation of the drive. It is to these ends that the present invention is directed.
The present invention provides an apparatus and method for improving operational performance of a disc drive by identifying defective servo burst patterns used to effect head positional control.
In accordance with preferred embodiments, a disc drive is provided with a head which is supported adjacent a surface of a rotatable disc on which servo data are stored. The servo data include a plurality of servo burst patterns which define a plurality of tracks on the surface of the disc.
A servo circuit, responsive to the head, controllably positions the head with respect to the surface of the disc in relation to a position error signal determined from a selected combination of burst signals transduced from the servo burst patterns. Additionally, the servo circuit identifies a selected servo burst pattern as defective in relation to differences between successive pairs of peak amplitude samples from the burst signal obtained from the selected servo burst pattern.
More particularly, the servo circuit preferably comprises a servo processor having associated programming to determine a sequence of the peak amplitude samples from the burst signal, determine a sequence of difference values in relation to differences between absolute values of successive pairs of the peak amplitude samples, and compare the sequence of difference values to a defect threshold. The servo processor identifies the selected servo burst pattern as defective when at least one of the difference values exceeds the defect threshold.
The defect threshold is preferably provided with a magnitude to differentiate between burst signals with random noise fluctuations, and burst signals affected by a thermal asperity or localized media anomaly in the associated servo burst pattern.
These and various features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.